Heart rate variability measures and the sympathovagal balance: differences between low and high fit subjects

The authors addressed the question wether differences in fitness are associated with differences in sympathetic tone, vagal tone, or sympathovagal balance, expressed in the Rosenblueth-Simeone factors m, n, and their product m/spl middot/n. They also determined whether fitness related differences in heart race variability (HRV) are associated with differences in autonomic tone or sympathovagal balance. They measured HRV, heart rate (HR), and, with autonomic blockade, the intrinsic HR and the Rosenblueth-Simeone factors in 21 low fit (mean/spl plusmn/SD age 27.5/spl plusmn/3.4 yrs; VO/sub 2/-max<50 ml/spl middot/kg/sup -1//spl middot/min/sup -1/, mean/spl plusmn/SD 39.8/spl plusmn/6.3 ml/spl middot/kg/sup -1//spl middot/min/sup -1/) and 23 high fit (mean/spl plusmn/SD age 24.1/spl plusmn/3.0 yrs; VO/sub 2/-max>50 ml/spl middot/kg/sup -1//spl middot/min/sup -1/, mean/spl plusmn/SD 57.6/spl plusmn/5.5 ml/spl middot/kg/sup -1//spl middot/min/sup -1/) healthy male subjects. HR and the intrinsic HR differed significantly between the low and high fit groups, but the sympathetic and vagal tone, and the sympathovagal balance were not significantly different. All time- and absolute frequency-domain HRV measures differed significantly. Linear regression of HR, intrinsic HR, sympathetic tone, vagal tone, sympathovagal balance, and the time- and frequency-domain HRV measures on VO/sub 2/-max values of all 44 subjects yielded significant correlations for resting HR, intrinsic HR, standard deviation of inter-beat-intervals, coefficient of variation, percentage of successive inter-beat-intervals differing more than 50 ms, respiratory sinus arrhythmia, and the very low frequency spectral HRV power. All correlations were below 0.50 (<0.25 explained variance). The authors´ results demonstrate that the fitness level alters neither sympathetic tone nor vagal tone, or the sympathovagal balance. At the same time, increased VO/sub 2/-max is associated with increased HRV.